This invention relates generally to the field of optical switches, and more specifically to systems for effecting a change in electrical current through the use of optically transmitted signals.
Regulations in the nuclear power industry require nuclear power plants to be built with four redundant divisions of safety equipment to monitor each parameter of plant operation, such as core temperature and coolant level. That is, for a given safety function, such as maintaining proper reactor core temperature, each of the plant's four safety divisions must independently sense the core temperature and determine whether it is within an acceptable temperature range. Thus, should a failure occur in as many as three of the four redundant safety divisions, an unsafe plant condition will still be detected by the remaining safety division, which will then take appropriate emergency measures.
A nuclear power plant operates in one of four basic modes: RUN, START UP/HOT STANDBY, REFUEL, and SHUTDOWN. Each of these modes must be monitored according to a different set of safety parameters, which must be simultaneously and independently monitored by each of the four safety divisions. It is therefore necessary for a single switch to simultaneously signal each of the four safety divisions when the plant is being changed to a new mode of operation, so that the safety parameters pertinent to the new mode can be monitored.
In order to assure that at least one of the redundant safety divisions will always be operational, nuclear safety regulations further require that each safety division be capable of operating independently of the others, so that no two safety divisions rely on a common sensor, electrical cable or circuit board. In addition to maintaining separate and independent components, the safety divisions must be electrically isolated from each other, so that no electrical components or connections from one safety division are located within 6 inches from those of another safety division. These requirements pose particular design problems at the mode selector switch of the reactor control panel, where all four safety divisions must meet to determine the present mode of plant operation so that corresponding safety procedures can be put into effect.
Prior to the present invention, control panels for nuclear reactors have utilized a manually operated 270.degree. rotary switch to select one of the four modes of plant operation. As this switch to select one of the four modes of mode to the next, a rotary shaft communicates the switch position to each of the four divisions of safety circuits, which are spaced along the shaft at intervals of 6 inches or more in order to provide adequate electrical isolation. This results in a rotary shaft over 18 inches long, which causes substantial problems resulting from tortional flexion and takes up a great deal of space in the control panel.
In order for the mode switch to work properly, it is essential that all four safety divisions be switched simultaneously when changing from one mode to the next. However, the use of a long rotary shaft in conventional rotary switches results in a high degree of twist along its length when the switch knob is turned, which can cause the safety division associated with the front end of the shaft to switch to the new mode before the other safety divisions. Such flexion may, for example, cause one of the safety divisions to be in the RUN mode while the remaining safety divisions are in the START UP mode, circumventing the redundancy which was intended to be an essential part of the safety system. Other disadvantages of the conventional switching arrangement are high space requirements behind the control panel needed to house the switch, high cost of construction, and frequent maintenance to clean the switch contacts and/or replace worn out mechanical parts.
The present invention uses optical data transmission to provide a method and apparatus which is particularly useful for switching electrical current in the safety divisions of a nuclear plant. It is compact, highly reliable, and provides effective electrical isolation of the safety divisions at their closest point of congregation, thus minimizing the possibility of short circuiting, electrical fires and other malfunctions.
It is therefore an object of the present invention to overcome the above-described problems of mechanical rotary switches, particularly those used to switch the safety divisions of a nuclear plant to a desired operational mode.
It is a particular object of this invention to provide a switching system which may be used in place of a mechanical rotary switch to eliminate the adverse effects of tortional flexion of the rotary shaft.
It is a further object to provide such a switching system in which all contacts associated with a particular switch position are simultaneously made.
It is another object to provide a compact switch which maintains a high degree of electrical isolation among the circuits connected to it, and specifically to provide such a compact switch which satisfies the electrical isolation requirements of safety regulations in the nuclear industry.
It is yet another object of the present invention to provide a switching system which is inexpensive to build and may be operated for long periods of time without maintenance.